1. Field of the Invention
The present invention relates generally to lifetime related quality evaluation of a semiconductor surface, and more particularly to a lifetime related quality evaluation method of a semiconductor surface of the type wherein a semiconductor wafer having a semiconductor thin layer over the main surface of a semiconductor substrate is evaluated for the lifetime related quality (or information) of the semiconductor thin layer and/or the vicinity thereof to achieve a non-contact, non-destructive quality evaluation of the semiconductor surface.
2. Description of the Related Art
A semiconductor device is formed on, or in the vicinity of, the surface of a semiconductor material (silicon, for example). In recent years, the production of high-density, semiconductor devices has promoted the demand for a wafer having an epitaxial structure (hereinafter referred to as "epitaxial wafer") to improve the defect-free characteristics of the surface layer.
In parallel with the progress of the high-density trend of the semiconductor devices, a further reduction of the thickness or thinning of the semiconductor surface layer has been promoted, which will require quality evaluation of the thin surface layer with higher accuracy. Several techniques of evaluating the quality of semiconductor materials are known, which include the wafer lifetime method, the MOS C-t method, and the photoluminescence method.
In the conventional wafer lifetime method, an electromagnetic wave is used to generate excited carriers in the wafer for subsequent detection of a transient phenomenon in which the excited carriers become ceased to exist. However, since the excited carriers are generated in a relatively deep area, a problem is that the quality evaluation of a semiconductor thin layer, such as one having the epitaxial structure, cannot be achieved.
Especially, in an epitaxial structure having a semiconductor substrate with a reduced resistivity to deal with a latch-up problem, i.e., an operation failure of semiconductor devices, majority carriers in the semiconductor substrate become extremely predominant with the result that the conventional wafer lifetime measurement cannot be achieved.
In the MOS C-t method, the surface of a semiconductor thin layer is oxidized to form an oxide film, followed by a process of forming a gate electrode to which is applied a gate voltage to form a depletion layer directly below the surface of the semiconductor thin layer for enabling detection of a transient phenomenon in which minority carriers are generated within the depletion layer.
This method, however, is a destructive inspection and hence wafers which have already inspected can not be used to actual device fabrication processes. Furthermore, a device and a process to be added to achieve evaluation render the evaluation by this method expensive and complicated in procedure.
Moreover, information obtained by this evaluation method is limited to one related to a region of the semiconductor thin layer having a depth of several .mu.m from the surface. Now considering that the thickness of the semiconductor thin layer ranges from several .mu.m to several tens .mu.m, a problem arises in that the evaluation method might be deemed insufficient to the equality evaluation of the semiconductor thin layer, on one hand, and cannot achieve evaluation of the interface between the semiconductor thin layer and the semiconductor substrate, on the other hand.
In the photoluminescence method, evaluation can be achieved in a non-destructive, non-contact manner. However, it is not clear about a depth to which extent the band edge emission attained by this method (namely, the emission at a wavelength of approximately 1.15 .mu.m at room temperature, as shown in FIG. 4) actually contains information.
Especially in the case of the wafers of the epitaxial structure, when the resistivity of the semiconductor thin layer is relatively low (not greater than 0.1 .OMEGA.cm), emission intensity is strong or high and hence can be evaluated in itself. However, when the resistivity of the semiconductor thin layer is relatively high (not smaller than 1 .OMEGA.cm), emission intensity is weak or low, and carriers generated within the semiconductor thin layer diffuse out into the semiconductor substrate and emit light therefrom. As a result, evaluation of the emitted light may contain information about the semiconductor substrate.
In the case where the resistivity of the semiconductor substrate is relatively high, a light emitting region becomes indefinite. Accordingly, it occurs likely that not only the quality of the semiconductor thin layer but also the quality of the semiconductor substrate are detected concurrently. Under these circumstances, evaluation with higher accuracy cannot be achieved.